• Regional cerebral blood flow (rCBF) was measured by the xenon 133 inhalation method in 60 patients with Parkinson's disease and compared with flow data obtained in 51 age-matched normal control subjects. Mean brain rCBF was significantly reduced in patients with parkinsonism (9.5%, P <.001). The most marked and significant rCBF decreases were observed in the older patients (18.8%, P <.001). There was no correlation between degree of rCBF reduction and duration of parkinsonism. Decreases in hemispheric mean rCBF values were similar in both hemispheres even in patients with unilateral signs. The present study provides additional evidence for involvement of the cerebral cortex in Parkinson's disease. The rCBF decline may be associated in part with high prevalence of mental impairment and cortical atrophy and with diminished cerebral metabolic rate due to brain dopamine deficiency in patients with parkinsonism.